a) Field of the Invention
The present invention relates to an imaging optical system for endoscopes.
b) Description of the Prior Art
Fiberscopes using image guide fiber bundles are practically employed in a large number as endoscopes. In the recent days, however, video endoscopes using various types of solid-state image pickup devices in place of the image guides are also used in a large number.
The video color imaging systems are classified roughly into the field sequential color system and the dot sequential color systems. The latter has a composition comprising a color encoding filter which is composed of mini-size color filters integrated in mosaic patterns corresponding to the picture elements of a solid-state image pickup device (usually referred to as the color mosaic filter). When a wide space is reserved between the filter and the light-receiving surface of the solid-state image pickup device, however, the light incident on the solid-stage image pickup device does not fall on the picture element on which the light is originally to fall after passing through the color encoding filter but on the neighboring picture element, thereby producing color ununiformity on an image (hereinafter referred to as the color shading). The color shading produced in this way cannot be avoided in an optical system of the same type as the optical system for endoscopes disclosed by Japanese Unexamined Published Patent Application No. 173415/62 wherein the principal ray l falls obliquely on an image pickup device I as shown in FIG. 1.